The thermal behavior and decomposition of polyphosphazenes have been studied in the art. See, for example, Bowmer et al., Macromolecules 1991, 24, 4827. At elevated temperatures, thermal behavior of phosphazenes depends on the types of side groups present. Specific side groups on the polyphosphazene can lead to crosslinking, or to depolymerization to cyclic oligomers. Both of these processes can affect the fire retardant properties of the polyphosphazenes. See, for example, Kourtides, D., J. Fire Sciences, 1983, 1, 200.
Phosphazenes are known for use as flame retardant additives in organic polymeric mixtures. Polymeric mixtures are obtained through curing reactions between the host organic polymer and functional side groups on the polyphosphazene. See, for example, U.S. Pat. No. 5,024,860. Polymeric mixtures which contain cyclic trimers and tetramers are known for use as flame retardant polyaramids, polyesters, and rayon. Except for rayon formulations, aryloxyphosphazenes are preferred due to their superior thermal stability. In all of these mixtures, the side groups on the phosphazene are chosen to maximize flame retardancy while providing miscibility and/or reaction with the host polymer. Flame retardancy is commonly attributed to generation of polyphosphoric acid and subsequent char formation, or to a modification of the decomposition mechanism of the polymer. See, for example, Weil, E. D., Encycl. Polym. Sci. technol. 1988, 10, 976.
One of the reasons phosphazenes have excellent flame retardant properties is the presence of phosphorus in the backbone. The art has made significant effort to incorporate phosphorus by use of small molecule or polymeric additives, copolymerizations, and chemical modification. The effectiveness of phosphorus compounds chemically bonded to an organic polymer varies with location--whether randomly distributed, isolated into blocks, or pendent to the main chain. See Reghunadhan Nair, C. P., et al., Polym. Deg. Stab. 1989, 26, 305. For example, the art shows phosphazenes bearing phosphate, phosphonate, and phosphine oxide side groups which utilize alkyl and alkoxide ligands. These phosphazenes, however, do not have satisfactory thermal stability.
A need therefore exists for polyphosphazenes which have improved thermal stability and which are useful as flame retardants in a wide range of polymers.